The invention relates to an electromechanical displacement device provided with a holder, an element displaceable relative to the holder, and an actuator connected to the holder and capable of displacing the element relative to the holder, which actuator is provided with at least a first and a second clamping member which are arranged separately from one another and can be clamped against the element, while at least one clamping member is provided with at least one clamping element of transducer material, and which actuator is further provided with a transport element of transducer material by which the distance between the clamping members can be changed while one clamping member is connected to the element with clamping force.
The invention further relates to an actuator suitable for use in such an electromechanical displacement device.
The term "clamping member" in the present Patent Application is understood to mean at least one surface which can be clamped against the element to be displaced in that the distance between the surface and the element can be reduced. A "series of clamping members" means at least two surfaces between which the element to be displaced can be clamped in that the distance between the surfaces can be reduced.
The term "transducer material" is understood to mean a material which changes its shape under the influence of a change in an electric or magnetic field, for example, piezoelectric materials, electrostrictive materials or magnetostrictive materials.
In an electromechanical displacement device known from U.S. Pat. No. 3,902,084 and depicted in FIGS. 1a-1h, an annular piezoelectric transport element 1 is connected on opposing sides to two annular piezoelectric clamping elements 3, 5 (two clamping members) which can be clamped against a shaft 7. The piezoelectric transport element 1 is connected to a holder 9.
The piezoelectric elements can be separately activated by a voltage source. Activation reduces the internal diameters of the clamping elements 3 and 5, while it increases the length of the transport element 1 in axial direction. In situation (a) depicted in FIGS. 1a-1h, all piezoelectric elements are deactivated. In situation (c), clamping element 3 is activated and clamps around the shaft 7. In situation (c), subsequently, transport element 1 is activated, whereby it increases in length, so that the clamping element 3 together with the shaft 7 is moved to the left. Then clamping element 5 is activated (d), so that the elements 3 and 5 are simultaneously clamped against the shaft. After this the clamping element 3 is deactivated (e) so that the clamping element 3 no longer clamps against the shaft 7. Subsequently the transport element 1 is deactivated (f), so that it decreases in length and the clamping element 5 together with the shaft 7 is moved to the left. The two elements 3 and 5 are activated in situation (g), and in situation (h) the clamping action of element 5 is discontinued, upon which the situation is identical again to the situation (b). To move the shaft further to the left, the steps (b) to (g) should be repeated as often as is necessary. The diameter changes of annular piezoelectric clamping elements which can be realised in practice are only small, so that high requirements are imposed on the accuracy of the shaft diameter, or else the clamping elements may be continually clamped around the shaft or may be incapable of being clamped around the shaft at all.
Temperature changes and the accompanying expansion of the shaft and the clamping elements are disadvantageous for a good clamping action of the displacement device. In the U.S. Patent mentioned above, accordingly, it is suggested to manufacture the shaft from a material which has the same coefficient of thermal expansion as the clamping elements. The clamping force between a clamping element and the shaft depends inter alia on the local diameter of the shaft, which is not constant owing to tolerances and which is therefore not exactly known. Furthermore, the device described in the U.S. Patent is sensitive to wear. If the internal diameter of the annular element increases by a few tenths of a .mu.m owing to wear, a good clamping action has already become impossible. Therefore, the displacement device must be manufactured to a high accuracy and be very wear-resistant. The greatest distance over which the element is displaceable is determined by the length over which the element has the required high accuracy. In view of manufacturing costs, this length will be chosen to be limited in practice, so that the distance over which the element can be moved is also limited.
The invention has for its object to provide a displacement device with which the said disadvantages are avoided.